Loop-switch sequence

A loop-switch sequence (also known as the for-case paradigm or Anti-Duff's Device) is a programming antipattern where a clear set of steps is implemented as a switch-within-a-loop. The loop-switch sequence is a specific derivative of spaghetti code.

It is not necessarily an antipattern to use a switch statement within a loop—it is only considered incorrect when used to model a known sequence of steps. The most common example of the correct use of a switch within a loop is an inversion of control such as an event handler. In event handler loops, the sequence of events is not known at compile-time, so the repeated switch is both necessary and correct (see event-driven programming, event loop and event-driven finite state machine).

This is not a performance antipattern, though it may lead to an inconsequential performance penalty due to the lack of an unrolled loop. Rather, it is a clarity antipattern, as in any non-trivial example it is much more difficult to decipher the intent and actual function of the code than the more straightforward refactored solution.

Example

An event-driven solution would implement a listener interface:

But without the listener, it becomes an example of the antipattern:

And here is the refactored solution: